The invention is directed to the field of activity sensors and, in particular, to an activity sensor for a heart pacemaker.
Piezoelectric activity sensors of the flexural type are known in the prior art (VIBRIT, piezoceramic of Siemens, Order No. N-281/5035, Pages 19 and 20). These sensors are composed of two piezoelectric wafers glued to one another which are usually oppositely polarized. Given this structure, the two exterior surfaces of the piezo-element are provided with an electrode. A center electrode also lies between the two wafers.
When this structure is bent, an electrical voltage arises between the two outer electrodes and is conveyed via terminals and serves as a measure for the activity acting on the piezo-element. The voltage is composed of a voltage via the upper piezoceramic wafer and of a voltage via the lower piezoceramic wafer. In one piezoceramic wafer, electrical voltage and polarization direction are isodirectional; in the other, by contrast, they are oppositely directed. These conditions can be understood when one considers that, when the piezoelement is bent, a mechanical tension arises, for example, in the upper piezoceramic wafer and, since the two wafers are joined, a pressure, i.e. a negative mechanical stress, thereby arises in the lower wafer.
It is known that piezoceramic can be depolarized on the basis of great mechanical stresses. The piezoceramic thereby entirely or at least partially looses its sensitivity as an activity sensor and becomes unusable for this application. The depolarization occurs in that the ceramic itself generates a correspondingly high electrical voltage given a great mechanical stress, this electrical voltage being directed opposite to the polarization. Given the above-described geometry, it is of no consequence in which direction the structure is bent. One of the generated electrical voltages will always be directed opposite the polarization in one of the wafers.
Alternatively, both piezoceramic wafers can be polarized in the same direction given such an activity sensor of the flexural type. In this case, the center electrode, i.e. the electrode between the two piezoceramic wafers, must be accessible for a connection. Usually, the two outer electrodes are interconnected and the voltage is taken between these and the center electrode. However, the output signal is thereby halved in comparison to a sensor having oppositely polarized piezoceramic wafers. At the same time, the capacitance becomes four times as high, so that the electrical energy remains constant. The risk of a mechanical depolarization is identical for both structures, regardless of the direction in which the structures are bent.
When the structure is to be always bent in only one direction, only one of the two piezoceramic layers can be utilized. The risk of a depolarization is thereby avoided. It is necessary, however, that the center electrode must be connected. This often represents a complication since this center electrode is frequently difficult to access. Here, too, only half the output signal derives and, over and above this, only half the electrical energy derives.
There is the further possibility that the piezoelement is composed of only a single piezoceramic wafer contacted at both sides and that this, for example, is glued to the inside wall of a heart pacemaker housing. Disadvantages corresponding to those in the two structures described above thereby occur.
A further disadvantage of all of these structures particularly derives when, for example, they are to be secured to a surface by gluing, as to the inside surface of a heart pacemaker housing. As long as this surface is conductive, there is the possibility of electrically connecting the electrode of the piezoelement that lies against this surface via the housing. When this electrode, however, is to be insulated from the housing or when the surface to which the piezoelement is glued is itself composed of insulating material, contacting difficulties arise.